Update documentation

lib/mailbox.dart

@@ -68,7 +68,8 @@ class Mailbox {
 
   /// Place a message into the mailbox if has space for it.
   ///
-  /// If mailbox already contains a message then [put] will throw.
+  /// If mailbox already contains a message or mailbox is closed then [put] will
+  /// throw [StateError].
   void put(Uint8List message) {
     final buffer = message.isEmpty ? nullptr : _toBuffer(message);
     _mutex.runLocked(() {
@@ -86,7 +87,7 @@ class Mailbox {
 
   /// Close a mailbox.
   ///
-  /// If mailbox already contains a message then it will be dropped.
+  /// If mailbox already contains a message then [close] will drop the message.
   void close() => _mutex.runLocked(() {
         if (_mailbox.ref.state == _stateFull && _mailbox.ref.bufferLength > 0) {
           malloc.free(_mailbox.ref.buffer);
@@ -101,8 +102,8 @@ class Mailbox {
 
   /// Take a message from the mailbox.
   ///
-  /// If mailbox is empty this will synchronously block until message
-  /// is available.
+  /// If mailbox is empty then [take] will synchronously block until message
+  /// is available. If mailbox is closed then [take] will throw [StateError].
   Uint8List take() => _mutex.runLocked(() {
         while (_mailbox.ref.state == _stateEmpty) {
           _condVar.wait(_mutex);